Photoelectric tube



March 9, 1937. K. H. KINGDON ET AL 2,073,522

PHOTOELECTRIC TUBE Filed NOV. 13, 1931 Inventors:

- Kenneth H. Kingdom Hugh E.Thomson',

f I Their Attorney.

Patented Mar. 9, 1937 UNITED STATES PATENT OFFICE.

- 2.073.522 rrlO'lOELECTRIC 'runn Application November 13, 1931', SerialNo. 574,804

1 6Claims. (cl. 250 -275);

rection that will intercept the tube. The im- The present inventionrelates to light-sensitive devices, more particularly to photo-electrictubes.

Objects of the present invention are to provide a light-sensitive deviceof extreme sensitiveness; 5 also one which presents an unusually largefield of view to a' light source and which responds to light reachingthe tube from any and all directions. Photo-electric tubes ordinarilyare made, in 10. two types: (1) Those in which the envelope is coated onthe interior with a silver layer which serves as a foundation for thelight-sensitive ma-- terlal, and (2) those in which the light-sensitivesurface is formed on an electrode physically separate from the envelope.I While these various forms of photo-electric tubes have givensatisfactory service inthe translation of light energy into electricalenergy, they are open to certain objections. The first meng tioned typehas the disadvantage of being opaque to visible light except when thereis an opening termed a window in the silver layer. The field of viewpresented by a tube of this type is thus limited to the area of thewindow, ordinarily quite small. The optimum size of such a window cannever exceed the greatest rectilinear,

dimension of the envelope, so that a tube'of this" sort responds tolight propagated only in certain directions.

' In the second typepof photo-electric tube, the

envelope is transparent to light and the lightsensitive' member iscontained within the tube physically separate from the envelope.Inasmuch as the position ofthe light-sensitive cathode is fixed, a tubeof this kind is also directive.

The disadvantages of these tubes are at once apparent. As anexample,assume it is desired to measure the intensity of rapidly moving sourcesof light, such as the heavenly bodies, over a 'considerable periodoftime, it is clear that the di-. rective. type of photo-electric tubemust be moved through the same angle as the moving light source if thelatter is tc be kept within the small held of view. Moreover, in thedetermination sources of large area over which the light intensity isnot uniform; for example daylight, a

photo-electric tube of limited viewmay give an erroneous indication onaccount of being respon lighted area.

In accordance with the objects of the present invention, we provide. atube which is, not open to the objection of being directive, but, on the56 contrary, responds tolight propagated in any diof theintegratedamdunt of light produced by sive to light emanated by only aportion of the proved tube thus presents an unusually large andunlimited field of view. The invention con-' templates 2.light-sensitive layer formed on the interior of the envelope, preferablyby a sputtering process, said layer being so thinas readily to allow theadmission of visible light, 1. e., radiations of wave lengths betweenabout 4000 Angstroms and 7000 Angstroms, through the envelope. It hasbeen found that a light-sensitive layer which is sufliciently'thin to betransparent .to visible light is more stable than layers oLordi narythickness; that is, the improved thin layer exhibits less tendencytoward having spontaneous changes occur in the structure of the activesurface, as is evidenced by the constancy of its.

photo-sensitivity;

In the drawing, numeral l designates an envelope of vitreous material,-such as glass or quartz, and preferably of spherical shape. The envelopeterminates at. the lower portion in a cylindrical elongation 2 which isprovided with a, rentrant stem 3. Within the envelope there is a pair ofelectrodes 4, 5 mounted rigidly in the stem. The electrode maytake theform of a washer of any base material, such as copper; plated withsilver and positioned flatwise above the stem by means of a verticalsupport wire 6. The electrode 5 constitutes a rod'or wire of stiffcharacter, arranged concentrically with the washer. There is a capsule 1positioned on either side of the .stem'by means of a framework securedto the rodelectrode. These capsules may contain pellets of an alkalimetal such-as caesium dlchromate or other activating material, togetherwith a suitablereducing' agent such as-silicon.

Theimproved tube may be manufactured in derstood that various changes inthe process and construction will suggest themselves to those. skilledin the art, without departing from the' spirit of the invention.Thesilvercoating on the washer 4 is first oxidized. This is convenientlyaccomplished by admitting oxygen into the envelope at a pressure oi.aboutmicrons of mercury and then glowing the'bulb byapplying thefollowing manner,-altho ughit is 'to be un-.

heated, preferably byinduced high ,irequency '1 currents, in order torelease the'caesium or otherreference character 9, has been shown..These conductors may be positionedv at opposite points on the bulb sothat the resistance of the lightsensitive material which spans theconductors maybe measured from time to time.. The resistalight-sensitive material. The caesium vapor reacts with the oxidizedwasher and forms thereon a light-sensitive layer. A glass pant-leg 8 maybe provided about the wire which. supports the washer, in order toprevent electrical leakage between the'electrodes 4 and 5;

'After the application of light-sensitive material to the oxidizedsurface of the washer, the bulb is baked out at a temperature of about300 C., for. a period of time which may vary from a few minutes to abouttwo hours, depending upon the amount of alkali metal present in thebulb, the latter being in the meantime connected to a suitableevacuating pump. In this manner all excess caesium or lightesensitivematerial that did not stick is driven out, leaving a thin layer oflight-sensitive material covering all the free' surface of the oxidizedsilver. The feature of removing'the excess alkali metal isdescribed andclaimed in the Bainbridge application, Serial No.

244,533, filed January 4, 1928, and entitled Photo-electric tube'sand amethod for preparing the light-sensitive surface therefor. Thisapplication has matured into Patent N9. 1,901,577, granted'March 14,1933. As stated in the said application, the removal of the excessalkali metal adds to the stability of operation and sensitivity of thelay .The' next step is to transfer the light-sensitive material from thewasher onto the envelope, and

this conveniently is accomplished by a sputtering operation. For thispurpose, the envelope is filled with an inert gas, such as argon, at apressure of about 100 microns, and a voltage of about 300, alternatingcurrent or direct current being.

appliedbetween the washer and the rod. In the case of direct current,the washer is made the cathode or negative member. Th'latter' is bombarded during, this glowing stage with argon ions,

40 which causes thelight-sensitivematerial to leave the washer and totravel toward the envelope, on which it condenses as an extremely thinsensitive layer extending overthe entire, perimeter of the envelopeinterior. .The duration of the sputteringprocess depends upon therequired thickness of layer," but we have found that when this layer isof minute thickness, in fact so thinas to be substantially transparentto light of wave lengths between about 4000Angstroms and 'z ooo Augstrozns, the sensitivity of the material may be. quite large, and at-thesame time stable, over long" periods of operation. The degree ofthinness made possible by this process in order to; obtain thesequalities lends itself to thefabrication of a photo-electric tube whichshows only a slight discoloration of envelope, and, therefore, isreadily transparent to visible light.- Thus, there is no need orproviding a window in the tube, as the entire ,envelope, serves thepurpose of 0 light.

In order accurately to determine when a sumcient amount oflight-sensitive material has collected on the envelope during thesputtering process,it may be desirable to provide a pair ofilush sealconductors, of which only one, designated by ance will be observed todecrease suddenly when the film is of suflicient thickness, and thesputtering treatment should be stopped immediately.

The flush seal conductors referred to may be retained and laterconnected together to constitute sensitive film. It will be we areunable to state the exact nature of the light-sensitive materialproduced on the envelope in this manner, it is probable that it consistsof a complex compound involving caesium,

.electrictubes provided with opaque films. While oxygen and silver.Regardless of the correctness of this belief, it is sufiicient to statethat this sensitive material may be derived by asputtering process froma base metal which has been silverplated, oxidized, and then coatedwithalight-v sensitive material such as caesium. It also ap-' pears that atransparent light-sensitive material having the minimum thicknessconsistent with a substantial photo-electric effect resistsphotode-activation with time to a marked extent. It. is possible thatthe material is so thin under the conditions stated that it is of fairlyuniformcomposition throughout, so that diffusion of material between thesurface and the body of the film does not ,cause the sensitiveness tochange with time.

The portion of the light-sensitive material which remains on the washer4 after the film has been formed on the envelope-does not deleterious--1y affect the operation of the device. This washer together with the rodelectrode may be connected together to constitute the positive electrodeof a circuit in which the photo-electric tube is-connectedso that anyphoto-electrons which normal- 1y would be produced at the washer by alight source are unable to leave, the washer. The

emanation of photo-electrons is thus confined to the transparent film oflight-sensitive material: From the foregoing, it is evident that aphotoelectric tube improved in accordance with our invention andprovided with a transparent photoelectric surface offers not onlyadvantages in its adaptation to certain uses which require an unlimitedfieid 01' view, but also offers a greater stability of operation, due tothe thinness of layer, thanphoto-electric tubes of the prior art.

- It isalso apparent that by reason of the spherical configuration ofthe light-sensitive cathode, the

improved photo-electric tube is particularlywell suited, from thestandpoint of minimum distortio'n,-to measure the intensity of lightsources which move over a large field of view.

-What we claim as newand desire to secure by LettersPatent of the UnitedStates, is:

1.- A photo-electric tube comprising a hermetie cally tight envelopecoated with light-sensitive material of which all the component partsare intimately mixed to form a homogenous layen.

isaid light-sensitive material covering an area within the envelopewhich is perimetrically complete and substantially transparent to light,and

a cooperating electrode in said envelope.

2. A photo-electric tube comprising a herinet icallytight envelopecoated with light-sensitive material, of which all the componentparts'are intimately mixed to form a homogeneous layer,

said light-sensitive material coverlngan area within the envelope whichis perimetrically com-* plete and tra sparent to light, said envelopealso containing a plurality of electrodes adapted to pass a dischargetherebetween.

3. In the art of fabricating a photo-electric tube having an envelope,the method which includes the steps of coating a foundation member insaid envelope with an oxidizable metal, oxidising said metal, thendepositing an alkali metal onto said envelope to form a light-sensitivefilm substantially transparent to light.

s. A photo-electric tube an enveiope', said envelope being coated on theinterior with a thin transparent mm of light-sensitive material, andcontaining a plurality of electrodes includinta rod member and a metalWasher, said washer surrounding said rod and being mounnormalwithrespecttotheazls oisaidrod.

6. A photo-electric tube comprising a hermetimaterial of which all thecomponent parts are intimately mixed to form .a homogeneous layer. themm and envelope together being substantialiv transparentto 1181 1 and acooperating electrode in said envelope.

. a. memoir.

HUGH; a. women.

0 cally tight envelope coated with light-sensitive

